Genetic Linkage
Chapter 5
I. Complete Linkage:
Genes are so close together as part of the same structure that they are
inherited as a single unit.
II. Incomplete Linkage
A. Genes are further apart, such that, at Prophase I during Pachytene,
cross-over events can separate original combinations.
B. This is a quantifiable event, because it produces new gamete types
that can be identified in a test cross.
C. The frequency of recombinant types in progeny show the frequency
of crossover events.
D. What influences crossover event frequencies?
1. Distance between genes
a. close - unlikely to crossover
b. apart - more likely to crossover
c. far apart - independent of each other (50 mu)
d. genes far apart on the same chromosome would not show
evidence of linkage in genetic crosses. Genes on the same
chromosome, whether or not they show linkage in genetic
crosses, are syntenic.
2. Use crossover frequencies to map distance between genes.
mu (cM) =Map Units or centiMorgans
III. Trihybrid Mapping: Three genes on one chromosome.
Each exhibits complete dominance. Create trihybrid heterozygotes
then conduct a test cross.
1. Parental? - most frequent = ABC, abc
these are genes that are in same chromosomes before
crossing over
2. Double crossovers? (DCO) - least frequent Abc, aBC
occur as products of the rarest crossover events
[double crossovers]
3. Distance for the single cross overs
4. Interference: Reduction in DCO events as genes come
closer together on the chromosome. Can calculate how
much interference is occurring by using the Coefficient of
Coincidence (C).
a. First: calculate the Expected Double Crossover
events = Predicted by multiplying the percentage of
total crossovers between each pair of genes.
f(DCOOBS) = DCO ¸ Total
f(DCOEXP) = f(SCO1) X f(SCO2)
b. Coefficient of Coincidence (C) = concept of interference
can be quantified by (C):
C = DCOOBS ¸ DCOEXP
I = 1 - C
I = interference
I = 1 when no DCO events occur (i.e. C=0)
I + = fewer DCO than expected (C less than 1)
I - = More DCO than expected (C greater than 1)
c. What interferes with crossover?
Hot spots
Crossover in one arm reduces probability of crossover nearby
IV. Linkage in Humans
A. Originally could only be done with X-chromosome
B. Recombination rate for autosomal genes in males and females
is often different
1. J.B.S. Haldane: organisms with a chromosomal mechanism of
sex determination, recombination is generally higher in the
homogametic sex.
2. 50% to 100% higher recombination in females than males in
the human genome.
3. In Drosophila, which also has homogametic females, no
recombination occurs in males; but in silk moth Bombyx mori,
which has heterogametic females, the females show no
recombination
C. Human Genome: 2,000 to 2,500 map units, spread over the 23
human chromosomes.
1. chromosomal abnormalities can give a clue to the location of genes
a. Trisomies used to locate genes in crops and Down's.
b. Alzheimer's syndrome gene localization was helped by the Down's
research
2. Deletion Mapping: technique through which genes can be located to
particular chromosome segment.
a. individual is heterozygous for a deletion on a given chromosome
that has a number of codominant genes.
b. If different alleles are present at each gene, each genotype should
be recognizable as a heterozygote
c. when a gene is in the deleted region a heterozygote should always
appear as a homozygote.
V. Somatic Cell Hybridization and The Human Gene Map
A. Somatic Cell Hybridization - 2 cells in culture can be induced to
fuse into a single hybrid cell. Example: Human and Mouse Cells
B. Heterokaryon: initial cell type with two nuclei. One from each cell.
C. Synkaryon: eventually the nuclei fuse. Over successive
generations in culture, chromosomes from one of the parent cells
are slowly lost.
Example: Usually the Human chromosomes are lost
D. Synteny Testing: create a battery of cell lines with different missing
chromosomes. Analyze the protein production of the different
cultures; use this to assign the protein to the chromosome.
VI. Four-Stranded Crossing Over
A. Not clear whether recombination took pace between chromosomes
before they replicated or after replication.
1. Attached X
VII. Use of haploid organisms in Linkage and mapping studies
A. Chlamydomonas and Neurospora: somatic cells are haploid, but
go through a diploid stage in reproduction.
B. The zygote divides meiotically to form 4 cells in the Neurospora asci
These haploid spores then divide mitotically to form ascospores.
C. Can map the gene to the centromere (cannot do crossing over since
the cells are haploid - no homologous chromosomes).
1. First Division Segregation (Meiosis I)
aa++
2. Second Division segregation (Meiosis II)
a+a+
+a+a
+aa+
a++a
VIII. Unequal Crossing Over
A. Duplication and deletion: mistake related to recombination
1. Homologous chromosomes sometimes mispair
2. Example: Bar in Drosophila melanogaster
3. Observed in heterozygote:
a. One allele has two wild type genes on either side of Bar
b. One allele has two mutant genes on either side of Bar
c. Analysis of progeny shows :
i. In normal pairing all progeny are the same as parents
ii. In mispairing and unequal crossing over, progeny appear that are
dissimilar to parents
B. Duplications and Deletions can be observed in changes in banding
patterns of chromosomes.
IX. Map Distances and Physical Distances
A. The map distance between two genes estimated by genetic crosses
generally corresponds with the physical distance.
1. Exceptions: centromeric region
2. Recombination may often be localized into particular regions. HOT
SPOTS
B. Restriction Fragment Length Polymorphisms (RFLP)